Method and apparatus for estimating and controlling initial time slot gain in a wireless communication system

ABSTRACT

In a time-division duplex (TDD) system, a reliable initialization scheme that is applicable to an automatic gain controller (AGC) at a base station is implemented in various forms depending on the availability of certain information such as signal-to-interference ratio (SIR), spreading factors and other parameters. A more accurate estimation of the initial control word of a gain-adjustable amplifier for one or more time slots is implemented. The scheme is applicable to AGC initialization for each time slot of the TDD system, but is also applicable to other systems of transmission, without limitation.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims priority from U.S. Provisional PatentApplication No. 60/458,347, filed Mar. 27, 2003, which is incorporatedby reference as if fully set forth.

FIELD OF INVENTION

This invention generally relates to an automatic gain controller (AGC)used in a wireless communication system. In particular, the inventionrelates to a reliable initialization scheme for estimating andcontrolling initial time slot gain control signals in a base station(BS) AGC.

BACKGROUND

In most wireless communication systems, the baseband signal at thereceiver needs to be converted from analog format into digital format sothat useful information can be recovered via a sequence of digitalprocesses. The common device that achieves this conversion is ananalog-to-digital converter (ADC). Given the number of output bits ofthe ADC, if the power of the input signal is too large, the output ofthe ADC may be saturated. On the other hand, if the power of the inputsignal is too small, the input signal may be severely quantized. In bothcases, the information to be recovered at the received end may be lost.A common approach to solve this problem is to apply a dynamicallyadjustable gain amplifier in front of the ADC so that the input signalof the ADC can be maintained at a desired level.

Typically, the adjustable gain is controlled using a closed-loopmechanism, as shown in FIG. 1, which is also called an AGC 100. The AGC100 includes two gain-adjustable amplifiers 105A and 105B, two ADCs 110Aand 110B, a digital signal power estimator 115, a power comparator 120and an accumulator 125. If the gain-adjustable amplifiers are controlledby an analog signal, a digital-to-analog converter (DAC) (not shown) maybe inserted between the accumulator 125 and the gain-adjustableamplifiers 105A and 105B, or the DAC may be incorporated within theamplifiers 105A and 105B.

The gain-adjustable amplifiers 105A and 105B amplify or attenuatein-phase signal I and quadrature baseband signal Q. Signal I is receivedvia an input 130 of gain-adjustable amplifier 105A and signal Q isreceived via an input 135 of gain-adjustable amplifier 105B. The ADCs110A, 110B convert the amplified or attenuated analog signals andconvert them into digital format.

Referring still to FIG. 1, the digital signal power estimator 115estimates the total power of the digital signal at the output of powerestimator 115. The comparator 120 compares the estimated digital signalinput power 140 output from power estimator 115 with a power referencevalue P_(ref) 145 and generates an error signal 150 which represents thedifference between the estimated input signal power 140 and the powerreference value P_(ref) 145. The error signal 150 is then accumulated bythe accumulator 125. Accumulator 125 outputs a gain control signal 155having a control word w which is used to control the gain of thegain-adjustable amplifiers 105A, 105B. The gain control signal 155depends not only on the power errors accumulated over time, but also onthe initial value of the accumulator 125.

In many conventional digital communication systems, the input signal ofan AGC is a continuous and smooth signal waveform. The power variationof the input signal is usually due to channel fading or power controland, therefore, such a power variation is slow relative to the dynamicsof the AGC loop. Under this condition, the AGC loop will operatecontinuously without any interruption and the initial value of theaccumulator is not important.

For digital communication systems using time division duplex (TDD)technology, there is a potentially large slot-to-slot variation of thereceived power, as shown in FIG. 2. This slot-to-slot power variation isprimarily due to the fact that the systems using TDD technology canflexibly change the number of data bursts, which are spread withdifferent codes and superposed together in each time slot, over time.Owing to this power variation, the AGC 100 is required to be able to setthe gain in front of the ADCs 110A, 110B quickly and correctly.Otherwise, the data at the beginning of the time slot may be lost due toeither saturation or severe quantization. To meet this requirement, theaccumulator 125 must be restarted with an accurate initial value suchthat an initial gain control signal is provided to the gain-adjustableamplifiers 105A, 105B at beginning of each time slot. The initial gainvalue of the gain-adjustable amplifiers 105A, 105B directly depends uponthe initial value provided by the accumulator 125.

What is needed is a method and apparatus to accurately determine theinitial value of the accumulator 125 at the beginning of each uplinktime slot at the base station, based on the level of informationavailable at the base station.

SUMMARY OF THE INVENTION

The present invention provides a reliable initialization scheme that isapplicable to an AGC. The scheme can be implemented in various formsdepending on the availability of certain information, such as asignal-to-interference ratio (SIR), spreading factors, and otherparameters.

Preferably, the present invention is implemented in a wirelesscommunication system including an automatic gain controller (AGC).Initial time slot gain control signals are estimated and controlled byinputting an initial input signal in front of a gain-adjustableamplifier for a j'th timeslot in a k'th frame. A control word w_(i)[k−1,j] is applied to the gain-adjustable amplifier at the end of the j'thtimeslot in the (k−1)'th frame. The initial input power is estimatedbased on the power measurement of the time slot j in the previous framek−1 by removing the power of code signals that will disappear from theincoming time slot, and adding the power of new code signals that willarrive in the incoming time slot.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred example, given by way of exampleand to be understood in conjunction with the accompanying drawingwherein:

FIG. 1 illustrates a conventional AGC arrangement;

FIG. 2 illustrates a TDD radio frame structure and power profile;

FIG. 3 illustrates an AGC operating in accordance with the presentinvention where P_(in) ⁰[k, j] is the initial input signal power infront of a gain-adjustable amplifier for the j'th timeslot in the k'thframe; and

FIG. 4 is a flow chart illustrating method steps implemented by the AGCof FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention will be described with reference to the drawingfigures wherein like numeral represent like elements throughout. Thefollowing description presents algorithms, with respect to differentlevels of assumption, to estimate the initial power control signal usingsuch information.

While the description that follows is specifically explained asapplicable to TDD and TDS CDMA (time division synchronous code-divisionmultiple access), it is to be noted that the invention in its broad formis also applicable to other systems of transmission, without limitation.

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base stationincludes but is not limited to a base station, Node-B, site controller,access point or other interfacing device in a wireless environment.

FIG. 3 shows an AGC 300 operating in accordance with the presentinvention. The AGC 300 is similar to AGC 100, except that it furtherincludes a slot clock 305, a frame clock 310 and an initial valuegenerator 315. The slot clock 305 generates a reset signal 325 and sendsit to the accumulator 125 at the beginning of each time slot. This resetsignal 325 is used to signal the accumulator 125 to replace the datakept in the memory of the accumulator 125 with a new value that is newlycomputed by the initial value generator 315 and sent to the accumulator125.

FIG. 4 is a flow chart of a method 400 implemented by AGC 300. In step405, the slot clock 305 generates an index (j) 330 of the current timeslot. In step 410, the frame clock 310 generates an index (k) 335 of thecurrent frame. The initial value generator 315 receives the index (j)from the slot clock 305 and the index (k) from the frame clock 310 andcalculates the initial control word w₀[k, j] in response to reset signal325. It is assumed that the j'th time slot is an uplink time slot.

In general, the rate of the slot clock 305 is N times higher than thatof the frame clock 310, where N is the number of time slots per frame.Using these indices, w₀[k, j] is denoted as the initial control word ofthe gain-adjustable amplifiers 105A and 105B for the j'th timeslot inthe k'th frame. In step 415, system information and existingmeasurements 320 associated with the current (j'th) time slot in the k−1and k-th frames is input into the initial value generator 315. At thebeginning of each time slot, the initial value generator 315 computesthe initial value of the accumulator 125 based on the system andexisting measurements 320 (step 420), which is the same as the initialcontrol word w₀[k, j] of the gain-adjustable amplifiers 105A, 105B, forthe current time slot (j'th) and current frame (k'th). In step 425, oncethe new initial control word w₀[k, j] is available, it is sent to theaccumulator 125. Upon receipt of the reset signal 325, the accumulator125 outputs the new initial control word w₀[k, j] to the gain-adjustableamplifiers 105A, 105B. The gain value of the gain-adjustable amplifiers105A, 105B is then reset (step 430). Ideally, if the initial controlword w₀[k, j] is computed accurately, the initial output power at thebeginning of the time slot will be very close to the reference powerP_(ref) 145. As a result, minimal adjustment will be needed at thegain-adjustable amplifiers 105A, 105B during the time period of thistime slot. Mathematically, this desired condition can be represented as:P _(ref) =G _(ADC)10^(bw) ⁰ ^([k, j]+α) P _(in) ⁰[k, j]  Equation 1where P_(ref) is the reference power (or the desired output power) ofthe ADCs 110A, 110B, G_(ADC) is the power gain of the ADCs 110A, 110B,P_(in) ⁰[k, j] is the initial input signal power in front of thegain-adjustable amplifiers 105A and 105B for the j'th timeslot in thek'th frame, w₀[k, j] is the initial control word w for the j'th timeslotin the k'th frame and 10^(bw+α) is the model of gain-adjustableamplifiers 105A and 105B. The variables b and α are characteristicparameters which are usually provided by manufacturers ofgain-adjustable amplifiers 105A and 105B as part of their respectiveunit specifications.

From Equation 1, the initial control word w₀[k, j] for thegain-adjustable amplifiers 105A and 105B can be expressed as$\begin{matrix}{{w_{0}\left\lbrack {k,j} \right\rbrack} = {\frac{1}{b}\left( {{\log\left( \frac{P_{ref}}{G_{ADC}{P_{i\quad n}^{0}\left\lbrack {k,j} \right\rbrack}} \right)} - \alpha} \right)}} & {{Equation}\quad 2}\end{matrix}$

On the right hand side of Equation 2, only the initial input signalpower P_(in) ⁰[k, j] is unknown. The present invention provides a methodfor estimating the initial input signal power P_(in) ⁰[k, j]. Theestimated P_(in) ⁰[k, j] is denoted as {circumflex over (P)}_(in) ⁰[k,j]. Then Equation 2 becomes: $\begin{matrix}{{w_{0}\left\lbrack {k,j} \right\rbrack} = {\frac{1}{b}\left( {{\log\left( \frac{P_{ref}}{G_{ADC}{{\hat{P}}_{i\quad n}^{0}\left\lbrack {k,j} \right\rbrack}} \right)} - \alpha} \right)}} & {{Equation}\quad 3}\end{matrix}$

The estimation accuracy of input power P_(in) ⁰[k, j] depends on theavailability of the system and measurement information 320. Thefollowing description presents algorithms with respect to differentlevels of assumption about the availability of the system andmeasurement information 320.

In a first preferred embodiment, it is assumed that a receiver (e.g., inthe base station (BS)) containing the AGC 300 knows all of theinformation about newly acquired data, and that the data that willdisappear in the upcoming timeslot. Since the AGC 300 is located at theBS of a TDD communication system, for example, the information, such asspreading codes and SIR about the arriving data and disappearing datamay be known at the BS. Using the above assumption, the presentinvention estimates the initial input power, based on the powermeasurement of this time slot j in the previous frame k−1 by (1)removing the power of the signals that will disappear from the incomingtime slot j of frame k and (2) adding the power of the new signals thatwill arrive in the incoming time slot j of frame k.

For example, let the number of appearing new codes be N₁. Let {SIR_(i)^(target)[k, j], i=1, . . . , N₁} be the target SIRs. Let {SF_(i), i=1,. . . , N₁} be the spreading factors for those codes in the j'thtimeslot of the k'th frame. Let the number of disappeared old codes beN₂. Let {SIR_(i) ^(measure)[k−1, j],i=1, . . . , N₂} be the measuredSIRs. Let {SF_(i), i=N₁+1, . . . , N₁+N₂} be the spreading factors forthose codes in the j'th timeslot of the (k−1)'th frame. In addition, let{overscore (P)}_(est)[k−1, j] be the average power measurement at theoutput of the ADC 110 and let {circumflex over (P)}_(interf)[k−1, j] bethe interference power estimation as an output of the channel estimationalgorithm for the j'th timeslot in the (k−1)'th frame, respectively.Then, the estimated input power for the j'th timeslot in the k'th frameis: $\begin{matrix}\begin{matrix}{{{\hat{P}}_{i\quad n}^{0}\left\lbrack {k,j} \right\rbrack} = {\frac{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack}{G_{ADC}10^{{{bw}_{l}{\lbrack{{k - 1},j}\rbrack}} + \alpha}}\quad\left( {{the}\quad{estimation}\quad{of}\quad{the}}\quad \right.}} \\{\left. {{{input}\quad{power}\quad{of}\quad{time}\quad{slot}\quad j},{{frame}\quad k\text{-}1}} \right) +} \\{\frac{C\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}{\sum\limits_{i = 1}^{N_{1}}\quad{{{SIR}_{i}^{target}\left\lbrack {{k - 1},j} \right\rbrack}/{SF}_{i}}}}{G_{ADC}10^{{{bw}_{l}{\lbrack{k,j}\rbrack}} + \alpha}}\quad\left( {the} \right.} \\{{{estimation}\quad{of}\quad{the}\quad{input}\quad{power}\quad{of}\quad{newly}\quad{arrived}}\quad} \\{\left. {signals} \right) -} \\{\frac{C\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}{\sum\limits_{i = 1}^{N_{2}}\quad{{{SIR}_{i}^{measure}\left\lbrack {{k - 1},j} \right\rbrack}/{SF}_{i + N_{1}}}}}{G_{ADC}10^{{{bw}_{l}{\lbrack{{k - 1},j}\rbrack}} + \alpha}}\quad} \\{\left( {{the}\quad{estimation}\quad{of}\quad{the}\quad{input}\quad{power}\quad{of}}\quad \right.} \\\left. {{disappearing}\quad{signals}} \right)\end{matrix} & {{Equation}\quad 4}\end{matrix}$where w_(i)[k−1, j] is the control word applying to the gain-adjustableamplifiers 105A, 105B at the end of the j'th timeslot in the (k−1)'thframe, and C is the constant term used in a WTRU transmit powercalculation for balancing a potential power offset. Applying Equation 4to Equation 3, the proper initial control words for the gain-adjustableamplifiers 105A, 105B are obtained as follows: $\begin{matrix}{{w_{0}\left\lbrack {k,j} \right\rbrack} = {{w_{l}\left\lbrack {{k - 1},j} \right\rbrack} + {\frac{10}{b}{{\log\left( \frac{P_{ref} - {C\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}{\sum\limits_{i = 1}^{N_{1}}\quad{{{SIR}_{i}^{target}\left\lbrack {{k - 1},j} \right\rbrack}/{SF}_{i}}}}}{\begin{matrix}{{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack} - {C\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}}} \\{\sum\limits_{i = 1}^{N_{2}}\quad{{{SIR}_{i}^{measure}\left\lbrack {{k - 1},j} \right\rbrack}/{SF}_{i + N_{1}}}}\end{matrix}} \right)}.}}}} & {{Equation}\quad 5}\end{matrix}$

AGC 300 operates in accordance with the present invention where{circumflex over (P)}_(in)[k, j] is the initial input signal power infront of the gain-adjustable amplifiers 105A, 105B for the j'th timeslotin the k'th frame. In a second preferred embodiment, it is assumed thatthe BS does not know all of the information that is needed. For example,the BS may not know the following information in advance:

-   -   a) which code will be disappearing,    -   b) the actual values of signal gain factors C for a newly        arriving code, but the maximum value of the factor C_(max) is        known, and    -   c) the spreading factors for each newly arriving code, but the        minimum spreading factors {SF_(i) ^(min),i=1, . . . , N₁} are        known.

Since it is preferred to overestimate the input power to prevent ADC 110saturation, the estimated input power can be given as: $\begin{matrix}{{{\hat{P}}_{i\quad n}\left\lbrack {k,j} \right\rbrack} = {\frac{C_{\max}\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}{\sum\limits_{i = 1}^{N_{1}}\quad{{{SIR}_{i}^{target}\left\lbrack {{k - 1},j} \right\rbrack}/{SF}_{i}^{\min}}}}{G_{ADC}10^{{{bw}_{0}{\lbrack{k,j}\rbrack}} + \alpha}} + {\frac{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack}{G_{ADC}10^{{{bw}_{l}{\lbrack{{k - 1},j}\rbrack}} + \alpha}}.}}} & {{Equation}\quad 6}\end{matrix}$

Now applying the above estimated input power into the Equation 3, theresult can be expressed as: $\begin{matrix}\begin{matrix}{{w_{0}\left\lbrack {k,j} \right\rbrack} = {{w_{l}\left\lbrack {{k - 1},j} \right\rbrack} +}} \\{\frac{10}{b}{{\log\left( \frac{\begin{matrix}{P_{ref} - {C_{\max}\quad{{\hat{P}}_{interf}\left\lbrack {{k - 1},j} \right\rbrack}}} \\{\sum\limits_{i = 1}^{N_{1}}\quad{{{SIR}_{i}^{target}\left\lbrack {k,j} \right\rbrack}/{SF}_{i}^{\min}}}\end{matrix}}{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack} \right)}.}}\end{matrix} & {{Equation}\quad 7}\end{matrix}$

Furthermore, if it is assumed that the BS has no information aboutC_(max) or {SF_(i) ^(min),i=1, . . . , N₁} or {SIR_(i) ^(target)[k, j],i=1, . . . , N₁} in advance, the control word, based on Equation 7,reduces to: $\begin{matrix}{{w_{0}\left\lbrack {k,j} \right\rbrack} = {{w_{l}\left\lbrack {{k - 1},j} \right\rbrack} + {\frac{10}{b}{{\log\left( \frac{P_{ref}}{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack} \right)}.}}}} & {{Equation}\quad 8}\end{matrix}$

Again, Equation 8 corresponds to overestimating the input power. It willgenerate smaller initial gain value for the gain-adjustable amplifiersand, therefore, prevent the ADC 110 from being saturated at thebeginning of each time slot.

The foregoing describes a preferred example of an initialization schemeas per the invention. While this invention has been particularly shownand described with reference to a preferred embodiment, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the scope of theinvention as described hereinabove.

1. In a wireless communication system including an automatic gaincontroller (AGC), a method of estimating and controlling initial timeslot gain in the AGC, the method comprising: (a) inputting an initialinput signal in front of the gain-adjustable amplifier for a j'thtimeslot in a k'th frame; (b) applying a control word w_(i)[k−1, j] tothe gain-adjustable amplifier at the end of the j'th timeslot in the(k−1)'th frame; and (c) estimating the initial input power, based on thepower measurement of the time slot j in the previous frame k−1, by: (i)removing the power of code signals that will disappear from the incomingtime slot; and (ii) adding the power of new code signals that willarrive in the incoming time slot.
 2. The method of claim 1 wherein theAGC is located in a base station.
 3. In a wireless communication systemthat does not have access to specific code-related information, thesystem including an automatic gain controller (AGC) having at least onegain-adjustable amplifier in communication with an analog-to-digitalconverter (ADC), a method for controlling the gain of thegain-adjustable amplifier, the method comprising, at the end of atimeslot j in a k−1 frame, applying a control word w₀[k, j] to thegain-adjustable amplifier wherein${{w_{0}\left\lbrack {k,j} \right\rbrack} = {{w_{l}\left\lbrack {{k - 1},j} \right\rbrack} + {\frac{10}{b}{\log\left( \frac{P_{ref}}{{\overset{\_}{P}}_{est}\left\lbrack {{k - 1},j} \right\rbrack} \right)}}}},$P_(ref) is the desired output power of the ADC, {overscore(P)}_(est)[k−1,j] is the average output power of the ADC, b is acharacteristic parameter of the gain-adjustable amplifier, andw_(i)[k−1, j] is the control word applied to the gain-adjustableamplifier at the end of the j'th timeslot in the (k−1)'th frame.
 4. Themethod of claim 3 wherein the AGC is located in a base station.
 5. Themethod of claim 3 wherein the specific code-related informationindicates which existing code will no longer appear in the nexttimeslot.
 6. The method of claim 3 wherein the specific code-relatedinformation includes the actual values of signal factors for a newlyarriving code.
 7. The method of claim 3 wherein the specificcode-related information includes spreading factors for a newly arrivingcode.
 8. In a wireless communication system that does not have access tospecific code-related information, the system including an automaticgain controller (AGC) having at least one gain-adjustable amplifiercoupled to an analog-to-digital converter (ADC), a method forcontrolling the gain of the gain-adjustable amplifier, the methodcomprising estimating the initial input power to the gain-adjustableamplifier, based on the power measurement of a time slot j in a previousframe k−1 by (1) removing the power of signals that will disappear fromthe incoming time slot and (2) adding the power of the new signals thatwill arrive in the incoming time slot.
 9. The method of claim 8 whereinthe AGC is located in a base station.
 10. An automatic gain controller(AGC) located in a wireless communication system, the AGC for estimatingand controlling initial time slot gain in the AGC, the AGC comprising:(a) at least one gain-adjustable amplifier; (b) an analog-to-digitalconverter (ADC) coupled to the gain-adjustable amplifier; (c) means forinputting an initial input signal in front of the gain-adjustableamplifier for a j'th timeslot in a k'th frame; (d) means for applying acontrol word w₀[k, j] to the gain-adjustable amplifier; and (e) meansfor estimating the initial input power, based on the power measurementof the time slot j in the previous frame k−1, by: (i) removing the powerof code signals that will disappear from the incoming time slot; and(ii) adding the power of new code signals that will arrive in theincoming time slot.
 11. The AGC of claim 10 wherein the means forapplying the control word w₀[k, j] comprises: (d1) an initial valuegenerator; (d2) a slot clock for outputting time slot j to the initialvalue generator; (d3) a frame clock for outputting frame k to theinitial value generator; and (d4) an accumulator coupled between theinitial value generator and the gain-adjustable amplifier, theaccumulator for receiving the control word w₀[k, j] from the initialvalue generator and applying the control word w₀[k, j] to thegain-adjustable amplifier.
 12. The AGC of claim 11 wherein the initialvalue generator receives system and measurement information.
 13. The AGCof claim 10 wherein the AGC is located in a base station.